1. Field of the Invention
The present invention relates to an electric furnace for refining ferrosilicon.
2. Description of the Prior Art
Heretofore, submerged arc type open electric furnaces have been prevailingly used in refining ferrosilicon, wherein a gas evolved in a reaction zone and consisting mainly of CO is contacted and burned with air at top level of charged raw materials to generate plenty of heat. Therefore, a heat-shielding hood has to be provided above an upper rim of a shell of the furnace to protect upper equipments of the furnace and to lead the burned gas out of a house which accomodates the furnace. The burned gas is sucked through a duct or the like, cooled, introduced in a dust collector such as a bag filter, and exhausted to the atmosphere after a removal of dusts.
In such a case, air for contacting with the CO gas rising from the reaction zone above the top level of the charged raw materials invades from a window arranged on an upright wall of the heat-shielding hood for poking the charged raw materials in the furnace as well as from a gap between a lower end of the upright wall and the upper rim of the shell. Said invaded air flows at first transversely on the top level of the charged raw materials and reaches around electrodes near the central area in the top level of the charged raw materials and contacts with the rising CO gas from the reaction zone thereby to burn the CO gas, and the burned gas is guided to a duct arranged at an upper part of the heat-shielding hood and discharged therefrom.
During the transverse flow, the invaded air cools the top level of the charged raw materials before its contact with the rising CO gas. While, the charged raw materials form firm crusts, because silica sintered at high temperature under an influence of high heat transmitted from the lower reaction zone is cooled from upward by the invaded air and SiO component contained in the rising CO gas deposits on the charged raw materials of lower temperature. The crusts gradually grow from the outer periphery to the central part of the top level of the charged raw materials and finally hanged at the top level of the charged raw materials and prevent the raw materials from descending. When such crusts have been formed, only the raw materials charged in a narrow zone of high temperature around the electrodes are melted and fall down into the reaction zone. As a result, a phenomenon occurs frequently that the raw materials are insufficiently supplied to the tip ends of the electrodes immersed in the raction zone.
Accordingly, in order to prevent formation of such crusts and hanging of raw materials, poking bars must be thrusted through a plurality of windows for poking arranged on the circumferential periphery of the upright wall of the heat-shielding hood and poke (so called "poking") the hanging crusts, thereby to break down and cause to descend the hanging crusts and the raw materials satisfactorily into the lower reaction zone. However, conventional methods of refining ferrosilicon have problems that thickness and length of the formed hanging crust become larger respectively owing to the cooling of the top level of the charged raw materials by the transverse flow of the invaded air and hence frequent and deliberate poking operations are always required, because, otherwise, supply of the raw materials into the reaction zone becomes insufficient quickly, the reaction zone is overheated by arc, gas blows of high temperature occur locally in the area where the raw materials are melted and fall away, and required electric power per unit weight of product is increased and yield of silicon is decreased due to increase of heat loss, sputtered silica stones and cokes and diffusion loss of SiO vapor etc.
The aforementioned poking operation has also drawbacks that it must be effected at a hot environment suffering from high radiation heat from the electric furnace, that areas to effect poking have become large due to scale-up of electric furnaces in recent years and long time is required in effecting poking operations at necessary areas in good timing, and that sufficient poking operations are difficult and refining efficiency decreases unavoidably due to blowout of hot gas during the poking operations.